Abdominal Aortic Aneurysm (AAA) is a common and devastating disease which is increasing in incidence. Although easy and inexpensive to detect by ultrasound, most aneurysms are small when detected and there is currently no medical regimen which will inhibit their growth. There is an increasing body of evidence implicating a family of matrix degrading enzymes, the matrix metalloproteinases (MMPs) in AAA. Although both MMP-9 and MMP-12 may have a role in AAA, we have identified a significant increase in total MMP-2 in AAA. Importantly, a larger proportion of the MMP-2 in AAA tissue is in the active form and is directly bound to the matrix suggesting ongoing proteolysis. In addition, we have demonstrated that AAA tissue contains increased levels of membrane type 1 MMP, the activator of MMP-2. We have also shown that doxycycline inhibits MMP-2 production by aortic smooth muscle cells in culture. We hypothesize that MMP-2, through its increased activation, has a central role in aneurysm formation and that this could be inhibited by doxycycline. This hypothesis will be examined through the following specific aims: 1. Determine the effects of individual MMPs implicated in AAA including MMP-2, MT1-MMP, MMP-9 and MMP-12 on the size and rate of aneurysm formation in a murine AAA model. 2. Determine the effects of doxycycline on the size and rate of aneurysm formation and progression in a murine model and correlate these effects with serum doxycycline levels. 3. Determine the mechanisms by which doxycycline down regulates MMPs in human aortic smooth muscle cells. Specific aim 1 will be accomplished using a mouse model of AAA characterized in our laboratory with four different knock-out mice, including MMP-2, MMP-9, MMP-12 and a TIMP-2 knock-out mouse in which activation of MMP-2 does not occur. Specific aim 2 will be accomplished by using doxycycline treatment in our murine model of AAA and correlating effects on aortic MMP expression, aneurysm size and growth rate with serum doxycycline concentrations. Specific aim 3 will be accomplished by determining MMP- 2 mRNA levels, mRNA half life, rate of mRNA transcription and identifying the doxycycline responsive elements in the MMP-2 promotor. The long term goal of this work is to develop pharmacologic therapies which specifically target MMPs important in aneurysm pathogenesis and progression.